N-monoalkylated alkylenediamines, and particularly N-monoalkylethylene diamines, especially where the alkyl is a cycloalkyl group, have found use as corrosion inhibitors, fuel additives, stabilizers for resins, and agents for the purification of acrylic monomers. To date no continuous methods for their preparation appear to have been reported. The preparative route to these materials commonly utilizes the reaction of a carbonyl compound (an aldehyde or ketone) with an alkylenediamine to afford the Schiff base, which is an imine, and subsequent hydrogenation of the imine functionality of the Schiff base to the corresponding amine, as is exemplified below by the reaction of cyclohexanone with ethylenediamine, ##STR1## where M.sup.O is a zerovalent metal used as a hydrogenation catalyst, such as nickel, palladium, platinum, and so forth. [Although the foregoing equation may be somewhat of a simplification of the actual reactions occurring it suffices for the purpose of pointing to the salient characteristics important to this discussion.]
The foregoing reaction is attended by the side reaction, ##STR2## which is significant for several reasons. Where one desires only pure monoalkylated product B the formation of the dialkylated material D now imposes a purification stage which in itself may prove cumbersome at best and which may substantially add to the cost of producing pure B, as for example by substantially decreasing the yield of pure monoalkylated product. In such a case one then may be faced with the conundrum of effecting the reaction under conditions of low conversion to avoid or to minimize dialkylated product formation, or one may push conversion to higher limits while forming more of the dialkylated product and necessitating more extensive purification.
The side reaction is also significant for its detrimental effect on the hydrogenation catalyst, especially where the process is conducted in a continuous mode. The diimine C is an excellent complexing agent for those metals which are commonly used as a hydrogenation catalyst, such as nickel and palladium. Consequently these metals are slowly but continuously leached, leading to a gradual loss of selectivity and activity as well as to metal loss. The leached metal may ultimately appear in the N-monoalkylated product where it also may be detrimental to product stability, or interfere with the intended use of the product.
We have found a means to avoid the foregoing problems and have devised a process for the preparation of a high purity N-monoalkyl alkylenediamine where the process is adaptable to a continuous mode. More particularly, we have found that if the alkylenediamine and the carbonyl compound are reacted in an aqueous system, the dialkylated product (i.e., N,N'-dialkyl alkylenediamine) is only sparsely soluble in the cooled product mixture and may be readily removed to thereby afford high purity (95+%) monoalkylated product, N-alkyl alkylenediamine, upon removal of unreacted materials from the product mixture. Secondly, we have found that platinum appears unique in its resistance to extraction by materials like C, the diimine, and consequently can be safely used without substantial loss in a continuous process. Our invention is grounded on these observations.